1. Field of the Invention
The present invention relates to a mobile communication method, which configures a DRX cycle (a discontinuous reception cycle) having an On duration, in which a downlink signal transmitted from a serving cell is to be monitored, and an Off duration other than the On duration, and a radio terminal.
2. Description of the Related Art
In the next generation communication system such as LTE (Long Term Evolution), in order to reduce the power consumption of a radio terminal, a technology called DRX (Discontinuous Reception) is employed (for example, TS36.321 V10.0.0).
In the DRX, a DRX cycle has an On duration, in which a downlink signal (for example, PDCCH: Physical Downlink Control Channel) transmitted from a serving cell is to be monitored, and an Off duration (Opportunity for DRX) other than the On duration. A radio base station transmits a dedicated signal addressed to a radio terminal only in an On duration of the radio terminal. As described above, the configuration is such that the radio terminal may monitor the downlink signal transmitted from the radio base station, only in the On duration, and may turn off its own receiver in the Off duration. In addition, the DRX cycle may include two cycles (a short DRX cycle and a long DRX cycle). Furthermore, a DRX mode may be configured in the state in which an RRC connection is established between the radio terminal and the radio base station (RRC connected state). That is, it should be noted that the Off duration of the DRX cycle is different from an RRC idle state. In addition, in the 3GPP standard, the long DRX cycle is mandatory and the short DRX cycle is optional.
However, in recent years, there have been increased radio terminals having various applications. The application is configured to periodically transmit and receive a predetermined message such as a keep-alive message or a state update message to/from a communication correspondent such as a server. In such a case, since a control signal is transmitted and received due to the transition toward an RRC state, whenever the predetermined message is transmitted or received, a shortage of a network resource will be caused.
In this regard, in order to suppress the shortage of the network resource, the provision of a DRX cycle (for example, an extended DRX cycle) longer than the existing DRX cycle (for example, the short DRX cycle and the long DRX cycle) (for example, RP-110454) is under consideration.
However, it is assumed that the length of the extended DRX cycle is several seconds or more and the extended DRX cycle is very long as compared with the short DRX cycle, the long DRX cycle and the like. Therefore, the Off duration of the extended DRX cycle is expected to be very long.
Thus, when the extended DRX cycle is configured, various countermeasures are necessary. For example, it is considered that normally, the radio terminal does not measure radio environments of a serving cell and an adjacent cell (hereinafter, referred to as an Intra-Frequency neighboring cell) operated with the same frequency as that of the serving cell in the Off duration, and therefore, if the Off duration is very long, then the radio environments may not be appropriately measured and reported.